Two similar automated analytical systems using liquid chromatography (LC) and microdialysis as an on-line sampling technique were applied to studies of enzyme kinetics. 2',3',5'-Triacetyl-6-azauridine (azaribine) with porcine liver esterase (PLE) and N-acetylphenylalanyl-3,5-diiodotyrosine (AcFY') with pepsin were used as model compounds. The microdialysis sampling technique permitted the rapid separation of low molecular weight analytes from macromolecules, thus simultaneously achieving clean-up of the samples and quenching of the reaction. The combination of rapid LC analysis and microdialysis sampling provided selectivity and automation. The systems are rugged and give reproducible results in agreement with those from manual sampling methods.